A phase-locked loop (abbreviation PLL) is an electronic circuit arrangement which influences the phase position and the frequency of the controllable oscillator via a closed control loop in such a manner that the phase deviation between an input signal of the phase-locked loop and an output signal picked up at the oscillator output is constant to a high degree. A signal of stable frequency and phase position is thus generated by means of a phase-locked loop.
The phase-locked loop is used in communications technology, control technology and measurement technology, for example, for the realization of frequency synthesizers, in digital communications systems, for clock recovery and for synchronization.
The simplest form of a phase-locked loop comprises a phase detector and a controlled oscillator which are combined in a control loop and influence one another mutually in this manner.
In the settled condition of the phase-locked loop, a tracking of the oscillator frequency and the phase relative to an input signal Sigin is therefore obtained. In the case of changes of the input signal Sigin or a change of the Sigout caused by an adjustment of the controlled oscillator, the phase-locked loop tries, determined by the negative feedback at the phase detector, to keep an error signal as small as possible and close to the value zero.
A controlled oscillator for a phase-locked loop is generally constructed with tunable capacitors. In high-frequency technology, varactors are used as tunable capacitors in order to vary the frequency of the controlled oscillator. For this purpose, a variable DC voltage is applied to the varactor, wherein the varactor behaves like a capacitor with capacitance dependent upon the DC voltage.
In semiconductor processes which are used for monolithic-integrated controlled oscillators, high quality varactors are generally unavailable. For such circuits, especially PN junctions of transistors or respectively channel capacitances of field-effect transistors are used as tuning elements. These monolithic-integrated semiconductor varactors provide a series of disadvantages.
Firstly, these monolithic-integrated semiconductor varactors behave in a very non-linear manner, in particular, so that a broad analogue tuning of the varactors over a tuning range is not practicable. Furthermore, especially semiconductor varactors realized on the basis of field-effect transistors provide a comparatively high 1/f noise, so that these monolithic-integrated semiconductor varactors add a significant noise component to the VCO output signal, and the quality of the output signal can be significantly impaired.
Furthermore, these monolithic-integrated semiconductor varactors generally provide only a small tuning range, so that the capacitance changes caused by the DC-voltage variations are too small for given applications. Accordingly, controlled oscillators with monolithic-integrated semiconductor varactors generally provide a very narrow bandwidth, so that the tuning range of the controlled oscillator amounts to only a few percent of the oscillation frequency. Broadband oscillators with very good phase noise, as required for measurement technology, cannot be realized with these monolithic-integrated semiconductor varactors.
Furthermore, varactors with a discrete construction are known, that is, as discrete components. Semiconductor diodes based on silicon, gallium arsenide or indium phosphide have so far been used for this purpose. Such varactors are also referred to as varicaps or respectively capacitance diodes. In view of their size and their rising manufacturing costs, these varactors are not suitable in increasingly miniaturized environments. Furthermore, there is the risk, that discrete varactors may be withdrawn by the manufacturer which leads to a redesign of the circuit. Accordingly, a cost-favorable and consistent manufacture of an electronic circuit by means of discrete varactors is not possible.
Furthermore, varactors using micro-systems technology (Micro-Electro-Mechanical-Systems, abbreviation MEMS-varactors) are known. In particular, these MEMS-varactors provide no 1/f noise. MEMS-varactors achieve a very good quality because of their low ohmic losses, for example, as a result of the absence of the bulk resistance present in the case of semiconductor varactors. The disadvantage with MEMS-varactors is their high sensitivity to vibrations and the Brownian noise of airborne and atmospheric molecules, so that, especially the microphony properties of such MEMS-varactors argue against using them as controllable oscillators.
High precision varactors based on MEMS technology, such MEMS-varactors, can be used, in particular, in voltage-controlled oscillators. These voltage-controlled oscillators can therefore also be used in phase-locked loops.
In order to obtain a high-precision MEMS-varactor, a varactor is embodied with a comb structure, which is biased differently via a plurality of DC voltages in order to generate a correspondingly precise capacitance. The manufacture of a varactor of this kind is very cost intensive, and the varactor can be realised only in very narrow-band applications.